This invention relates to a circuit for measuring a source resistance of a sensor. In particular, this invention provides for measurement of the source resistance of the sensor by applying a known test current to the sensor to obtain substantially stable voltages, and measuring the resulting voltages to calculate the source resistance.
Continuous measurement of electrodes in a solution is known throughout the art. For example, Blackmer in U.S. Pat. No. 3,661,748 discloses a device in which an ac signal is applied via an electrode to the conductive fluid in which the electrode system is disposed. An ac signal detector is connected to the dc circuitry to measure the ac current flow. A threshold circuit responsive to the output of the ac signal detector indicates fault in the electrochemical sensor system when the output is of a predetermined magnitude. The system measures a change in resistance of the electrode membrane by a phase detector using the ac signal source as a phase reference. A resistance threshold is provided such that an alarm will sound when the threshold is exceeded.
Connery et al., U.S. Pat. No. 4,189,367 disclose another system for testing electrodes. In Connery et al., the electrodes comprise a glass membrane pH electrode and a reference electrode. The membrane is tested for damage during periodic testing periods by applying a test current through the electrodes and measuring the corresponding changing voltages produced between the electrodes. A reverse current of the same magnitude and duration is then applied through the electrode system to discharge capacitance. The test system does not test the reference electrode and pH electrode separately.
While it is important to monitor sensor electrodes while the sensing system is in use, it is desirable to do this quickly, with low power consumption, and without corrupting the data signal. Some prior art systems are very slow because capacitors are used to measure resistance. Very large resistances have large RC time constants which makes measurement slow. Many prior art systems are not properly isolated or shielded, thus allowing stray capacitances to effect the values being measured.
Previous prior art systems have also disclosed that bulk resistances of a membrane, such as a glass membrane, can be measured by passing a unidirectional test current through the glass and measuring the resultant voltage drop to calculate the resistance. A unidirectional current would not be acceptable in a continuously monitoring system such as the present invention because the electrode would be polarized and would produce erroneous results.